1. Field of the Invention
The present invention relates to a shock absorbing device for a steering column, and more particularly to a shock absorbing device for effectively absorbing and relieving shock applied to a steering column in the initial stage of a car collision, thus effectively preventing the shock from being transmitted to a steering wheel and thereby protecting a driver in the event of a collision.
2. Description of the Prior Art
FIGS. 1 and 2 show the construction of a typical shock absorbing device for a steering column. As shown in the drawings, the steering column comprises a cylindrical upper column 10 and a cylindrical lower column 20, one end of the lower column 20 being interferentially fitted into the upper column 10 to form a shock absorbing device in the interferential fitting junction between the columns 10 and 20. The shock absorbing device includes a plurality of lengthwise lugs 21, which are axially formed with regular spacing on the fitting part of the lower column 20. Due to the lengthwise lugs 21, the lower column 20 is able to be interferentially fitted into the upper column 10. The lengthwise lugs 21 are preferably formed on the lower column 20 by a pressing process.
The upper and lower columns 10 and 20 are respectively mounted to upper and lower portions of a car body using mounting brackets (not shown). In the event of a collision, the lower column 20 frictionally retracts into the upper column 10 so that the junction between the internal surface of the upper column 10 and the lengthwise lugs 21 of the lower column 20 absorb and relieve the collision impact applied to a steering wheel through the steering column. The shock absorbing device thus protects the driver from such a collision impact.
FIG. 3 is a chart showing the impact absorbing characteristic of a typical shock absorbing device of a steering column during a car collision. As shown in FIG. 3, the collision impact load represented in units of kilogram force (kgf), which is absorbed by the junction between the internal surface of the upper column 10 and the lengthwise lugs 21 of the lower column 20, is rapidly increased, reaching a peak point within a very short time after the collision initiates. After reaching the peak point, the absorbed impact load is constant at about 200 kgf, due to a relative displacement (stroke) between the upper and lower columns 10 and 20. Such peaking of the absorbed impact load is caused by the fact that the upper column 10 is only minutely deformed in the initial stage of the retraction of the lower column 20 into the upper column 10, thus preventing the lower column 20 from easily retracting into the upper column 10. This phenomenon results from a relative displacement between the columns 10 and 20 which is extremely small at the peak point of the load. Such a small displacement between the two columns 10 and 20 in the initial stage of the collision means that the collision impact is directly applied to the steering wheel through the steering column.
Therefore, the conventional shock absorbing device for a steering column fails to rapidly and effectively absorb the collision impact in the initial stage of a car collision, and transmits the impact to the steering wheel through the two columns and in turn to the driver. Such a shock absorbing device is ineffective in protecting the driver in the event of a collision.
As described above, the impact load resulting from a car collision is constant after the peak point. However, since a collision impact is instantaneously generated upon collision, the interferential fitting junction between upper and lower columns requires a rapid deformation characteristic to effectively absorb the impact in the initial stage of the collision.
Another problem experienced by a typical shock absorbing device is that it is very difficult to obtain a predetermined size of the lengthwise lugs 21 from the pressing process. Therefore, it is difficult to effectively control the impact load. The steering column with such a shock absorbing device is also problematic in that the upper and lower columns 10 and 20 are welded to a car body, causing a welding strain in each column. Due to this such welding strain, the junction between the internal surface of the upper column 10 and the external surface of the lower column 20 may be deformed, failing to maintain the designed structure of the junction. Therefore, the impact load, which can be effectively absorbed by the shock absorbing device in the case of a collision, cannot be accurately determined.